Measurement and control of the CO.sub.2 concentration in laboratory cell-culture incubators is most commonly accomplished by means of a thermal conductivity detection system. The thermal conductivity cell, or detector, is a differential thermometer, set up as an electronic bridge circuit that is balanced to be equal, with two thermistors in a common block or metallic housing to add thermal stability. (See U.S. Pat. No. 3,929,584, ((Mansfield)). This cell is most commonly placed in a working environment that is isolated from the surrounding area, but is not restricted to its placement. In some instances, the detector cell is located in an air flow path external from the working environment, but contiguous with the environment. One thermistor sensor is enclosed in the block/housing and detects chamber temperature only. The other thermistor sensor is exposed to the chamber environment. The measured difference between the two thermistor sensors is the thermal conductivity (density) of the atmosphere, or its ability, when moved at an even rate, to remove the small amount of heat from the exposed sensor. If all other factors remain constant and only the carbon dioxide content is varied, the "TC" cell output (when properly calibrated) will indicate changes in CO.sub.2 concentration. Unfortunately, the TC cell is affected by barometric pressure, temperature, humidity and the velocity of air flow past the sensor cell. These variables are controlled or compensated for with the use of electronic zeroing circuitry to compensate for changes in temperature and relative humidity levels. In monitoring the effects of CO.sub.2 in an atmosphere, absolute humidity must be held constant so any change in thermal conductivity is caused only by a change in the CO.sub.2 concentration. Under the worst circumstances, a change in absolute humidity can cause such a significant change in thermal conductivity that the controller can shift the CO.sub.2 content by as much as 4%.
To maintain a stable humidity level in laboratory incubators, a pan of water is placed within the working environment and its temperature allowed to equilibrate. The incubator, working atmosphere, must reach a point of near saturation in order to maintain an absolute humidity level that will not change with ambient conditions.
For the laboratory investigator that does not want to operate their incubator in a saturated condition, but does want accurate CO.sub.2 control, the drifting of the thermal conductivity sensor's reference becomes a problem with regard to the accuracy of the CO.sub.2 gas concentration in the incubator. That is, as the absolute humidity changes, so does the reference base of the CO.sub.2 gas sensor.
When operating a dry incubator, as opposed to a saturated one, ambient humidity fluctuations will effect CO.sub.2 zero calibration. Since the fluctuations possible in extreme ambient temperature changes have less effect on the total absolute humidity, the CO.sub.2 calibration can be affected as much as 11/2% in the worst case which does not represent as severe a problem, but does create an error that could prove critical in the pH level of the cell media being cultured within the incubator working chamber.
The present inventor has developed a unique system for detecting or measuring CO.sub.2 gas concentration in an enclosed environment using a thermal conductivity sensor without the inaccuracies of the prior art systems which are caused by fluctuations in absolute humidity. The present invention removes the inaccuracies due to the effect of changes in absolute humidity levels on the zero calibration point of the thermal conductivity sensor which is used to measure and control the CO.sub.2 content within any controlled atmosphere (e.g., a cell-culture CO.sub.2 incubator). This is accomplished by having the output of the thermal conductivity detector multiplexed with the output of an absolute humidity detector via firmware in a microprocessor control system. The output from the microprocessor controller is a humidity-corrected signal that will maintain a stable reference output with respect to constantly changing absolute humidity levels in the controlled atmosphere. By constantly correcting this output due to changes in absolute humidity levels, accuracy and errors were reduced from a high of 4% for conventional systems to less than 0.2% for the present invention. These errors are primarily caused by manually or automatically zeroing the thermal conductivity detection system in a potentially unstable environment. This zeroing is typically conducted by the product user or non-compensating type control system.
The present invention is also capable of specifically measuring and controlling the CO.sub.2 content in an enclosed (e.g., a laboratory CO.sub.2 incubator) by means of a thermal conductivity detector, where the relative humidity level may be allowed to equilibrate to that of surrounding ambient conditions as well as conditions that may be increased to virtual saturation.
The two factors that contribute to inaccuracies in a thermal conductivity gas control system, i.e., relative humidity and dry-bulb temperature, are taken into consideration in the measurement system of the present invention to arrive at the absolute humidity compensated carbon dioxide gas concentration.
The present invention also provides many additional advantages which shall become apparent as described below.